Throttle Body Guide - Air Apparent

Understanding throttle bodies

Richard Holdener

July 29, 2011

How much power is a throttle body upgrade worth? The question seems simple enough, but the answer is somewhat less so. Power gains seem to range from a little as 0 hp to as much as 25-30 hp. It seems like a pretty broad range of power gains for a simple bolt-on.

In point of fact, we are here to tell you that a throttle-body swap netted over 60 hp on a Kenne Bell supercharged application. In reality, the question is not so much can a throttle body swap net a sizable jump in power, but rather, does such an upgrade always results in said gains. The quick and dirty answer is a resounding no, but the reason behind it is certainly worth a closer look.

In its simplest form, the throttle body is nothing more than an air valve. There is no magic to the workings of throttle body, though there is some magic to maximizing the flow rate through it. A given opening will flow a certain amount of air, but radiused entries, thin throttle blades, and the elimination of hardware in the air stream all combine to further increase the airflow of a given bore size. It stands to reason that a 90mm throttle body should outflow an 80mm throttle body, but it is possible for a well-designed 80mm throttle body to outflow a poorly designed 90mm.

Back in the early days of the 5.0L Mustang, we marveled at the ability to upgrade to a 65mm throttle body. Modern motors are equipped with 90mm and dual 75mm throttle bodies right from the factory. Contrary to popular belief, a larger throttle opening does not reduce low-speed power, but the effect on throttle response can be dramatic. The reason is that (compared to a smaller throttle body), any throttle position will offer a sizable increase in airflow. Opening a 90mm throttle body even 3-5 percent is like opening a smaller throttle body 10-12 percent. This makes part-throttle driving difficult as minor throttle angle changes result in dramatic power differences.

Now that we have established that a larger throttle body will outflow a smaller version (assuming equal design quality), we can take a look at why the installation of a larger throttle body may or may not improve power. An example works well here. According to testing performed at Kenne Bell, the stock '05-up Three-Valve dual 55mm throttle body flows 890 cfm. Using the formula that 1 hp requires 1.5 cfm, we see that this stock throttle body might support as much as 593 hp, or way more than the factory-rated 300 hp.

The first obstacle in terms of power production is the fact that the throttle body is not the only component in the induction system. The flow rate of the throttle body is only as good as the supporting components. In the case of the '05-up GT, the flow rate of the complete induction system is only 509 cfm, a significant drop from the 890 cfm offered by the throttle body alone.

According to our formula, the stock air intake system is capable of supporting 339 hp, or just slightly more than the stock power output. According to this data, power gains will come from the air intake system and not the throttle body on a stock or mildly modified motor. Of course, installation of a better exhaust and revised cams can increase efficiency and allow more power than those number would suggest.

Another important factor when it comes to the power gains offered by the throttle body is engine combination. From the most basic standpoint, the higher the power output of the test motor, the larger the throttle body required. Upgrading a throttle body already capable of supporting 600 hp with a larger version capable of supporting 700 hp on a 300hp motor will have predictable results.

The 600hp throttle body is already oversized for the application, so there is no need to upgrade. This is especially critical on supercharged applications, where elevated power levels are more commonplace. You'd be hard pressed to find many 600hp normally aspirated Three-Valve 4.6L motors, but add a supercharger to the mix and they are everywhere.

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It should be noted that throttle body sizing and maximum flow is less critical on a blow-through (centrifugal supercharger and turbo) application than a draw-through (positive displacement). Pressurizing the air before the throttle body artificially increases the flow rate of the throttle body. Obviously it is best to eliminate any pressure differential caused by the throttle body, but a stock throttle body will be less of a hindrance on a blow-through than a draw-through.

One of the areas often overlooked on a draw-through supercharged application is the induction system. The thought process seems to be that boost cures everything, and as long as there is boost, everything is working just fine. The reality is that the inlet system is a critical element on a twin-screw or Roots supercharged application.

Restrictions from the air-intake system (including the throttle body) into the blower result in a drop in flow, boost, and power production out of the blower. One of the confusing facts is that the power loss is present despite an increase in boost (and power). The consensus is that if the boost goes up, there must not be a restriction. The reality is actually that losses associated with a restrictive inlet system (throttle body, MAF, air intake, and even intake manifold between the throttle body and supercharger) increase with increased boost pressure and power output.

Testing on a Kenne Bell supercharged Three-Valve motor illustrate that a throttle body upgrade on a 500hp application (8 psi on stock motor) was worth 20 hp. Performing the same test at 615 hp was worth 35 hp and an amazing 61 hp at 20 psi. The greater the boost and power, the greater the losses associated with a restrictive throttle body. Remember, the same throttle body upgrade that was worth 61 hp on the supercharged application wasn't worth any power on the normally aspirated motor!

Uncovering all the valuable data on the correlation between airflow, horsepower, and associated losses is easy at the proper test facility. Equipped with a Dynojet chassis dyno, Superflow airflow bench, and extensive data-logging equipment, the Kenne Bell facility was ideally suited for gathering data, but what about the average Joe?

As luck would have it, testing for an inlet restriction can be performed with nothing more than a simple (and inexpensive) vacuum gauge. There is a direct correlation between the amount of vacuum present in the inlet system and the loss in boost and power offered by that restriction. Hooking up a vacuum gauge in front of the supercharger (in the inlet manifold on a Kenne Bell supercharged application) will immediately indicate any inherent restrictions. Vacuum present in the air intake system at WOT is an indication of a restriction.

According to (exhaustive) testing performed by Kenne Bell on the engine dyno, chassis dyno, and airflow bench, 1 inch HG of vacuum (vacuum is measured in inches; boost in pounds) is roughly equivalent to 0.5 psi of boost. On a high-horsepower Kenne Bell application, it is not uncommon to see 4-5 inches of vacuum using the factory air intake and throttle body. This restriction is limiting boost (and attending power output) by 2-2.5 psi. Curing the inlet restriction would add 2-2.5 psi of boost, and what supercharged Mustang owner wouldn't want an extra 2.5 psi of boost?

For the slide rule crowd, there is (naturally) a mathematical correlation between the restriction and power losses. Using a previous example, we saw that replacing the throttle body on a 500hp supercharged Three-Valve was worth 20 hp. Data logging indicated that the combination produced 502 hp at 8.5 psi with the stock throttle body. Upgrading to the Stage 2 air-intake kit, which included a 130mm, oval throttle body and revised air intake system (the combination increased the total airflow of the air intake system by 200 cfm), increased power output to 526 hp.

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The airflow improvements offered by the Stage 2 system decreased the vacuum (inlet restriction) present from 2.22 inches to 1.33 inches. Minimizing the inlet restriction resulted in an increase on boost pressure from 8.5 psi to 8.9 psi. If we take the change in boost offered by the upgrade 0.44 psi and divide it by the original boost (8.5 psi), we see that boost increased by 5 percent (0.44/8.5 = 5 percent). If we take that 5 percent and multiply it by the original power number (502 hp), we get 25 hp (matching the power gain almost exactly). This percentage gain in power relative to the boost and vacuum loss was consistent up to 20 psi.

We have concentrated our efforts primarily on the throttle body and air intake system, but the intake manifold connecting the throttle body to the inlet of the supercharger is equally important. The largest, fastest, most powerful supercharger in the world can be severely limited by a poorly designed inlet system, including the intake manifold. The intake design is almost always a compromise, as it must maximize airflow while simultaneously fitting the tight confines of the engine bay. Given the rear entry of most twin-screw and Roots-style Mustang superchargers, it becomes difficult to package the intake manifold and supercharger in such close proximity to the firewall.

Recognizing the importance of inlet restrictions, Kenne Bell designed its Mammoth intakes to maximize flow for high-horsepower applications (available for a variety of popular Ford modular engines). As with the throttle body and air intake systems, vacuum present in the intake manifold will reduce both boost and power. Testing on the Mammoth intake revealed an increase of 56 hp at 22.8 psi of boost. Like the throttle body, the gains offered by the Mammoth intake manifold increased with the power output.